Base station using reference signal power control

ABSTRACT

A wireless spread spectrum base station has a plurality of modems. The modems produce at least one baseband channel signal and a baseband reference signal. At least one forward power controller controls a power level of the at least one baseband channel signal. A baseband signal combiner combines the at least one baseband channel and baseband reference signals. A radio frequency transmitter modulates to radio frequency and transmits the combined signal. A reference power control processor determines a desired transmit power level of the baseband reference signal to the desired transmit power level.

BASE STATION USING REFERENCE SIGNAL POWER CONTROL

This application is a continuation of U.S. patent application Ser. No.10/176,276, filed on Jun. 20, 2002, now U.S. Pat. No. 6,542,719, whichis a continuation of U.S. patent application Ser. No. 10/046,025, filedon Oct. 29, 2001, now U.S. Pat. No. 6,456,828, which is a continuationof U.S. patent application Ser. No. 09/904,021, filed on Jul. 12, 2001,which issued on Mar. 19, 2002 as U.S. Pat. No. 6,360,079, which is acontinuation of U.S. patent application Ser. No. 09/665,865, filed onSep. 20, 2000, which issued on Jan. 22, 2002 as U.S. Pat. No. 6,341,215,which is a continuation of U.S. patent application Ser. No. 09/196,808,filed on Nov. 20, 1998, which issued on Jan. 30, 2001 as U.S. Pat. No.6,181,919, which is a continuation of U.S. application Ser. No.08/797,989, filed on Feb. 12, 1997, which issued on Nov. 24, 1998 asU.S. Pat. No. 5,842,114.

BACKGROUND

1. Field of the Invention

The present invention relates generally to wireless local loop andcellular communication systems. More particularly, the present inventionrelates to a wireless communication system which dynamically adjusts thepower of signals transmitted over reference channels from a base stationto minimize power spillover to adjacent communication cells.

2. Description of the Related Art

Wireless communication systems have rapidly become a viable alternativeto wired systems due to their inherent advantages. Wireless systemsenable subscribers to freely move throughout the operating range of aservice provider and even into the territory of other service providerswhile using the same communication hardware. Wireless communicationsystems are also utilized for applications where wired systems areimpractical, and have become an economically viable alternative toreplacing aging telephone lines and outdated telephone equipment.

One of the drawbacks with wireless communication systems is the limitedamount of available RF bandwidth. There is a constant desire to improvethe efficiency of these systems in order to increase system capacity andmeet the rising consumer demand. A factor that degrades the overallcapacity of wireless communication systems is signal power spilloverbetween adjacent cells or base stations. This occurs when the power ofsignals transmitted by a base station in a particular cell exceeds theboundary of that cell, otherwise known as the operating range. Thespillover becomes interference to adjacent cells and degrades theefficiency of the system. Accordingly, minimizing spillover is one ofthe most important issues in wireless communications system design.

Forward power control (FPC) is used to minimize spillover by adjustingthe power level of signals transmitted from the base station tosubscriber units on assigned channels. The FPC operates in a closed loopwherein each subscriber unit continuously measures its receivedsignal-to-noise ratio and transmits an indication back to the basestation of whether the base station should increase or decrease thetransmit power to that subscriber unit. The closed loop algorithmassists in maintaining the transmit power level from the base station ata minimum acceptable level, thereby minimizing spillover to adjacentcells.

FPC, however, cannot adjust the power level for reference channels suchas the pilot signal, broadcast channel or paging channel. Since there isno closed loop algorithm that operates on these channels, the referencechannel transmit power level for the worst case scenario is typicallyused. The power level is generally more than what is required for mostsubscriber units, resulting in spillover to adjacent cells.

There have been prior attempts to overcome the problem of spillover.U.S. Pat. No. 5,267,262 (Wheatley, III) discloses a power control systemfor use with a CDMA cellular mobile telephone system including a networkof base stations, each of which communicates with a plurality ofsubscriber units. Each base station transmits a pilot signal which isused by the mobile units to estimate the propagation loss of the pilotsignals. The combined power of all base station transmitted signals asreceived at a mobile unit is also measured. This power level sum is usedby the mobile units to reduce transmitter power to the minimum powerrequired. Each base station measures the strength of a signal receivedfrom a mobile unit and compares this signal strength level to a desiredsignal strength level for that particular mobile unit. A poweradjustment command is generated and sent to the mobile unit whichadjusts its power accordingly. The transmit power of the base stationmay also be increased or decreased depending upon the average noiseconditions of the cell. For example, a base station may be positioned inan unusually noisy location and may be permitted to use a higher thannormal transmit power level. However, this is not performed dynamically,nor is the power correction based upon the total transmit power of thebase station.

Accordingly, there exists a need for an effective method for controllingthe power level of reference channels transmitted from a base station.

SUMMARY

A base station for use in a wireless spread spectrum communicationsystem produces at least one channel signal and a reference signal. Theat least one channel and reference signals are combined. The combinedsignal is transmitted. A desired transmit power level of the referencesignal is determined using a transmit power level of the combinedsignal. A transmit power level of the reference signal is adjusted usingthe desired transmit power level.

BRIEF DESCRIPTION OF THE DRAWING(S)

FIG. 1 is a communication network embodying the present invention;

FIG. 2 is the propagation of signals between a base station and aplurality of subscriber units;

FIG. 3 is a base station made in accordance with the present invention;and

FIG. 4 is a flow diagram of the method of dynamically controlling thetransmit power of reference channels in accordance with the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

The preferred embodiment will be described with reference to the drawingfigures wherein like numerals represent like elements throughout.

A communication network 10 embodying the present invention is shown inFIG. 1. The communication network 10 generally comprises one or morebase stations 14, each of which is in wireless communication with aplurality of fixed or mobile subscriber units 16. Each subscriber unit16 communicates with either the closest base station 14 or the basestation 14 which provides the strongest communication signal. The basestations 14 also communicate with a base station controller 20 whichcoordinates communications among base stations 14 and between basestations 14 and the subscriber units 16. The communication network 10may optionally be connected to a public switched telephone network(PSTN) 22, whereupon the base station controller 20 also coordinatescommunication between the base stations 14 and the PSTN 22. Preferably,each base station 14 is coupled with the base station controller 20 viaa wireless link, although a land line may also be provided. A land lineis particularly applicable when a base station 14 is in close proximityto the base station controller 20.

The base station controller 20 performs several functions. Primarily,the base station controller 20 provides all of the operation,administration and maintenance (OA&M) signaling associated withestablishing and maintaining the communications between the subscriberunits 16, the base stations 14 and the base station controller 20. Thebase station controller 20 also provides an interface between thewireless communication system 10 and the PSTN 22. This interfaceincludes multiplexing and demultiplexing of the communication signalsthat enter and exit the system 10 via the base station controller 20.Although the wireless communication system 10 is shown as employingantennas to transmit RF signals, one skilled in the art should recognizethat communications may be accomplished via microwave or satelliteuplinks. Additionally, the functions of a base station 14 may becombined with the base station controller 20 to form a master basestation. The physical location of the base station controller 20 is notcentral to the present invention.

Referring to FIG. 2, the propagation of certain signals in theestablishment of a communication channel 18 between a base station 14and a plurality of subscriber units 16 is shown. Forward signals 21 aretransmitted from the base station 14 to a subscriber unit 16. Reversesignals 22 are transmitted from the subscriber unit 16 to the basestation 14. All subscriber units 16 located within the maximum operatingrange 30 of the cell 11 are serviced by that base station 14.

Referring to FIG. 3, a base station 100 made in accordance with thepresent invention is shown. The base station 100 includes an RFtransmitter 102, an antenna 104, a baseband signal combiner 106 and areference channel power control (GCPC) algorithm processor 108. The basestation 100 also includes a plurality of modems 110, one for eachchannel, for generating a plurality of assigned channels 112 and aplurality of reference channels 114. Each modem 110 includes associatedcode generators, spreaders and other equipment for defining acommunication channel as is well known by those skilled in the art.Communications over assigned and reference channels 112, 114 arecombined by the combiner 106 and upconverted by the RF transmitter 102for transmission. The power of each assigned channel 110 is individuallycontrolled by the FPC. However, in accordance with the presentinvention, the power of the reference channels 114 is simultaneously anddynamically controlled by the GCPC processor 108.

The total transmit power of all channels 112, 114 is measured at the RFtransmitter 102 and this measurement is input into the GCPC processor108. As will be described in detail hereinafter, the GCPC processor 108analyzes the total transmit power of all channels 112, 114 andcalculates the desired transmit power level of the reference channels114. Preferably, the power level is measured prior to outputting the RFsignal to the antenna 104. Alternatively, the power level may be: 1)measured at the combiner 106; 2) sampled at each assigned and referencechannel 112, 114 and summed; or 3) received as an RF signal just aftertransmission using a separate antenna (not shown) co-located with thebase station antenna 104. Those skilled in the art should realize thatany method for monitoring the total transmit power at the base station100 may be employed without significantly departing from the spirit andscope of the present invention.

Dynamic control of the power of reference channels 114 is performed byusing several assumptions in analyzing the total transmit power. It isassumed that the FPC for the assigned channels 112 is working ideallyand the power transmitted to each subscriber unit 16 is adjusted so thatall subscriber units 16 receive their signals at a particularsignal-to-noise ratio. Since changing the transmit power to a particularsubscriber unit 16 affects the signal-to-noise ratio seen at othersubscriber units 16, the analysis of transmit power by FPC for eachassigned channel 112 is preferably performed continuously.Alternatively, the analysis may be performed on a periodic basis, asappropriate, to adjust the power for each assigned channel 112.

Prior to the analysis of the total transmit power, several factors mustbe defined: γ denotes the signal-to-noise ratio required at a subscriberunit 16, N_(o) the white noise power density, W the transmit bandwidthand N the processing gain. The propagation loss is such that if thetransmit power is P, the power level P_(r) of a subscriber unit 16located at distance r is:P _(r) =P*β(r)  Equation (1)Different propagation models may be utilized depending upon the size ofthe cell, such as a free space propagation model, a Hata model or abreak-point model. Those of skill in the art should realize that anyempirical or theoretical propagation model may be used in accordancewith the teachings of the present invention. For example, the free spacepropagation model is used in small cells. In this model the propagationloss is:

$\begin{matrix}{{\beta(r)} = \frac{\alpha}{r^{2}}} & {{Equation}\mspace{14mu}(2)}\end{matrix}$where

$\begin{matrix}{\alpha = \frac{\lambda^{2}}{( {4\pi} )^{2}}} & {{Equation}\mspace{20mu}(3)}\end{matrix}$and λ is the wavelength of the carrier frequency. Accordingly, if thetransmit power is P, the power seen at distance r is inverselyproportional to the square of the distance. Thus, the power P_(r) seenat distance r is:

$\begin{matrix}{P_{r} = {P*\frac{\alpha}{r^{2}}}} & ( {{from}\mspace{14mu}{Equations}\mspace{14mu} 1\mspace{14mu}{and}\mspace{14mu} 2} )\end{matrix}$

When the FPC is operating on assigned channels 112, the powertransmitted P_(i) from the base station 100 to a subscriber 16 that islocated at a distance r_(i) from the base station 100 is:

$\begin{matrix}{P_{i} = {{\frac{N}{N + \gamma}{a( r_{i} )}} + {\frac{\gamma}{N + \gamma}P_{T}}}} & {{Equation}\mspace{14mu}(4)}\end{matrix}$where P_(T) is the total transmit power and:

$\begin{matrix}{{a( r_{i} )} = \frac{\gamma\; N_{0}W}{N\;{\beta( r_{i} )}}} & {{Equation}\mspace{14mu}(5)}\end{matrix}$

Since a reference channel 114 must be received adequately throughout theoperating range 30 of the cell 11, the transmit power requirement P_(G)for a reference channel 114 becomes:

$\begin{matrix}{P_{G} = {{\frac{N}{N + \gamma}{a(R)}} + {\frac{\gamma}{N + \gamma}P_{T}}}} & {{Equation}\mspace{14mu}(6)}\end{matrix}$where R is the operating range 30 of the cell 11. The value of a(R) canbe calculated easily for any propagation model. Accordingly, P_(G) is aconstant plus a fraction of the total transmit power P_(T). Since thetotal transmit power P_(T) is continuously monitored at the base station100, the reference channel transmit power P_(G) is updated dynamicallyinstead of transmitting it for the worst case, which corresponds to themaximum transmit power P_(T) that the base station 100 can transmit.

For example, for the aforementioned free space propagation model, thepropagation loss is:

$\begin{matrix}{{\beta(r)} = \frac{\alpha}{r^{2}}} & ( {{from}\mspace{14mu}{Equation}\mspace{14mu} 2} )\end{matrix}$where

$\begin{matrix}{\alpha = \frac{\lambda^{2}}{( {4\pi} )^{2}}} & ( {{from}\mspace{14mu}{Equation}\mspace{14mu} 3} )\end{matrix}$and λ is the carrier frequency of the signal. In this model, at distancer_(i):

$\begin{matrix}{{\beta( r_{i} )} = \frac{\alpha}{r_{i}^{2}}} & ( {{from}\mspace{14mu}{Equation}\mspace{14mu} 2} )\end{matrix}$and

$\begin{matrix}{{a( r_{i} )} = {\frac{\gamma\; N_{0}W}{{\alpha\; N}\;}{r_{i}^{2}.}}} & ( {{from}\mspace{14mu}{Equations}\mspace{14mu} 2\mspace{14mu}{and}\mspace{14mu} 5} )\end{matrix}$

Substituting R for the operating range 30 of the cell 11:

$\begin{matrix}{\;{{{a(R)} = {\frac{\gamma\; N_{0}W}{\alpha\; N}R^{2}}},}} & ( {{from}\mspace{20mu}{Equations}\mspace{14mu} 2\mspace{14mu}{and}\mspace{14mu} 5} )\end{matrix}$we have

$\begin{matrix}{P_{G} = {{\frac{\gamma}{\gamma + N}\frac{N_{0}W}{\alpha}R^{2}} + {\frac{\gamma}{\gamma + N}P_{T}}}} & ( {{from}\mspace{14mu}{Equation}\mspace{14mu} 6} )\end{matrix}$

Therefore, using the free space propagation model, the optimum referencechannel transmit power is given by a constant term, which isproportional to the square of the cell radius, plus a variable termwhich is a function of the total transmit power P_(T).

The significance of the present invention can be further illustrated bythe following numerical example. Suppose system parameters are given as:

-   -   γ=4 (desired signal to noise ratio)    -   N=130 (processing gain)    -   W=10×10₆ (transmit bandwidth)    -   N₀=4×10⁻²¹ (white noise density)    -   R=30×10³ m (30 km cell radius)    -   λ=0.1667 m (corresponding to 1.9 GHz carrier frequency).        Using the free space propagation model:

$\begin{matrix}{\alpha = {\frac{(0.1667)^{2}}{( {4\;\pi} )^{2}} = {1.76 \times 10^{- 4}}}} & ( {{from}\mspace{14mu}{Equation}\mspace{14mu} 3} )\end{matrix}$

Therefore, when the total power P_(T) transmitted from the base stationis 100 watts, the reference channel transmit power P_(G) should be:

$\begin{matrix}\begin{matrix}{P_{G} = {\lbrack \frac{4}{130 + 4} \rbrack \times \lbrack \frac{4 \times 10^{- 21} \times 10 \times 10^{6}}{1.76 \times 10^{- 4}} \rbrack \times}} \\{( {3 \times 10^{4}} )^{2} + {\lbrack \frac{4}{130 + 4} \rbrack \times 100}} \\{= {3\mspace{14mu}{Watts}}}\end{matrix} & ( {{from}\mspace{14mu}{Equation}\mspace{14mu} 6} )\end{matrix}$

Referring to FIG. 4, the method 200 for dynamically controlling thereference channel transmit power P_(G) is shown. Once all of the systemparameters have been defined (step 202) and several constants arecalculated (β(R), a(R)) (step 204), the processor 108 then calculates Aand B, which are used to determine the reference channel power levelP_(G) (step 206). The total transmit is power is measured at the basestation 100 (step 208) and the desired reference channel power level PGis calculated (step 210) using the formula:P _(G) =A+B*P _(T)  Equation (7)Once the desired reference channel power level P_(G) is calculated (step210), all of the reference channels 114 are set to the calculated powerlevel (step 212). This process is then repeated (step 214) tocontinually monitor the total transmit power at the base station 100 todynamically control the power level of the reference channels 114.

The required transmit power for a reference channel 114 can change by asmuch as 12 dB depending on the traffic load of the cell 11. As a result,in an application where the reference channel power level P_(G) is setsuch that it is sufficient under the highest traffic load expected(i.e., worst case), the reference channel transmit power level P_(G)will exceed the required power level necessary most of the time. Themethod of the present invention controls the reference channel transmitpower level P_(G) optimally by reducing it when the traffic load islight and increasing it when the traffic load is high such that reliablecommunications are maintained at all times. In this manner, thespillover to neighboring cells is kept at minimum possible levels andoverall system capacity is increased.

Although the invention has been described in part by making detailedreference to certain specific embodiments, such details is intended tobe instructive rather than restrictive. It will be appreciated by thoseskilled in the art that many variations may be made in the structure andmode of operation without departing from the spirit and scope of theinvention as disclosed in the teachings herein.

1. A wireless spread spectrum base station comprising: a plurality ofmodems for producing at least one baseband channel signal and a basebandreference signal; at least one forward power controller for controllinga power level of the at least one baseband channel signal; a basebandsignal combiner for combining the at least one baseband channel andreference signals; a radio frequency transmitter for modulating to radiofrequency and transmitting the combined signal; a reference powercontrol processor for determining a desired transmit power level of thebaseband reference signal using a transmit power level of the combinedsignal; a power controller associated with the baseband reference signalfor adjusting a transmit power level of the baseband reference signal tothe desired transmit power level.
 2. The base station of claim 1 whereinthe at least one channel signal is a plurality of channel signals. 3.The base station of claim 1 wherein the desired transmit power level isdetermined using the combined signal transmit power level and a distanceto an edge of the cell.
 4. The base station of claim 1 wherein thedesired transmit power level is determined using a free spacepropagation model.
 5. The base station of claim 1 wherein the desiredtransmit power level is determined using a data propagation model. 6.The base station of claim 1 wherein the determined reference channeltransmit power is determined using a break point propagation model. 7.The base station of claim 1 wherein the combined transmit power level ismeasured at the radio frequency transmitter.
 8. The base station ofclaim 1 wherein the combined transmit power level is determined at thesignal combiner.
 9. The base station of claim 1 wherein the combinedsignal transmit power level is determined at the modems.
 10. The basestation of claim 1 wherein the combined signal transmit power level ismeasured at an antenna external to the base station.
 11. The basestation of claim 1 wherein the at least one forward power controllercontrols the power level using a closed-loop algorithm.